The present invention relates to a signal detection circuit. More particularly, the invention relates to a signal detection circuit for indicating synchronism. The invention relates to a signal detection circuit which identifies with less and reduced misjudgments, the condition of synchronism in a phase synchronization circuit having a loop consisting of a phase detector, a loop filter and a voltage controlled oscillator.
A conventional measure for identifying the condition of synchronism and asynchronism or the existence of signals in a phase synchronization circuit utilizes a synchronism establishing signal detection circuit, as hereinafter described. In a phase synchronization circuit having a phase detector, a loop filter and a voltage controlled oscillator, the phase of the output oscillations of the voltage controlled oscillator is synchronized with the phase of the input signals by controlling the voltage controlled oscillator with the difference between the phase of the input signals and the phase of the output oscillations. A second phase detector is added to the circuit. Output signals of the voltage controlled oscillator fed through a 90.degree. phase shifter and the input signals of the phase synchronization circuit are fed to the second phase detector. A level judgment circuit comprising a low pass filter and a comparator or differential amplifier is connected to the output of the second phase detector and provides synchronism indicating signals from the output of the second phase detector.
In the known synchronism indicating signal detection circuit, the output of the low pass filter shows a characteristic of saturation starting from 0 to 1 when the phase synchronization circuit is pulled in at the input signal phase. Therefore, by setting a specific judgment level between 0 level and 1 level in the level judgement circuit, the condition of synchronism, asynchronism and the presence of signals are identified.
The known synchronism idicating signal detection circuit has a disadvantage, however. The disadvantage is that when low signal to noise signals, that is, signals with noise, are supplied as the input signals of the phase synchronization circuit, the level judgment circuit becomes unstable for level regulation due to the noise and the probability of misjudgment becomes high.
The principal object of the invention is to provide a signal detection circuit having less and reduced misjudgments in determining synchronism and asynchronism.
An object of the invention is to provide a signal detection circuit of simple structure having a level judgement circuit having less and reduced misjudgments in determining synchronism and asynchronism of a phase synchronization circuit.
Another object of the invention is to provide a signal detection circuit which functions with efficiency, effectiveness and reliability to indicate synchronism and asynchronism conditions of a phase synchronization circuit with minimal misjudgments or misdeterminations.